This invention pertains to an apparatus and a method for determining the gas entrainment characteristics of liquids, such as lubricating compositions, particularly automatic transmission fluids (ATFs).
The intermixing of a lubricant and air results in the formation of a gas-in-liquid dispersion. Such gas-in-liquid dispersions are technically termed "foams" (see for example: Perry's Chemical Engineers' Handbook, Sixth Edition, McGraw-Hill Co., New York, 1984, Chapter 18). In the lubricant art, the terms air "entrainment" and "foaming" are commonly applied to describe the processes of air incorporation into, and release from, lubricants, respectively. The formation of a stable air-in-oil dispersion in a lubricant under pressure is commonly referred to as "air entrainment", whereas foaming is normally understood to mean the release of dispersed air from a liquid phase at atmospheric pressure. The bubble rich layer which forms on top of the liquid phase is referred to as "foam." Foaming, as used in this context, is the result of poor release of air from the liquid phase. Normally, an air-in-oil dispersion, a lubricant with entrained air formed under pressure, will foam when the pressure is released and the dispersion is returned to atmospheric pressure.
Foaming is a function of the interfacial tension of the dispersion and the viscosity of the liquid phase. Air entrainment, as was previously noted, is the ability of the lubricant to form a stable air-in-oil dispersion under pressure. Air entrainment characteristics are influenced by the interfacial tension of the dispersion (as are foaming characteristics), and are also highly dependent on the size of the bubbles formed and the ability of the lubricant to stabilize the bubbles either through viscous isolation or additive adsorption. The amount of air that can be entrained by a lubricant is controlled by many factors, as indicated above. What is of interest is that lubricants vary in the maximum amount of air that they can entrain irrespective of the volume of air introduced into them. The ability to determine this maximum air entrainment capacity of a lubricant is the subject of this invention.
In many lubrication applications air and the lubricant are mixed together at high shear rates and at high temperatures. These conditions cause formation of air-in-oil dispersions. Entrained air in lubricants is detrimental to performance for several reasons. First it disrupts the lubricant film, thereby leading to wear. Second, air in intimate contact with the lubricant under pressure and at elevated temperatures causes oxidation. And third, air is compressible so hydraulic fluids with entrained air give soft or "spongy" responses. Transmissions (manual, automatic or continuously variable) are particularly susceptible to air entrainment. These devices have rotating elements that dip into the lubricant and in the process effectively mix air into the lubricant. Transmissions of this type also have pumps such that any air drawn into the pump will be dispersed into the lubricant phase. This aerated fluid then is subject to the problems previously described.
Many apparatuses and tests are known for determining the foaming characteristics of lubricants, including the ASTM D-892 Standard Test Method for Foaming Characteristics of Lubricating Oils, and the General Motors DEXRON.RTM.-III Foam Test, GM-6297M. However, there are no such apparatuses or procedures for determining the air entrainment characteristics of lubricants.
The present invention provides an apparatus and a method to accurately determine the gas entrainment characteristics of liquids, such as new or used lubricants, particularly automatic transmission fluids.